Empty tin in tank (6)
I believe the answer is:
vacant
'empty' is the definition.
(similar in meaning)
'tin in tank' is the wordplay.
'tin' becomes 'can' (synonyms).
'in' is an insertion indicator.
'tank' becomes 'vat' (I've seen this before).
'can' going within 'vat' is 'VACANT'.
(Other definitions for vacant that I've seen before include "Empty, unoccupied" , "Unoccupied - unthinking" , "Not occupied, perhaps apartment" , "available" , "unemployed" .)